1. Field of the Invention
The present invention relates to a surface emitting laser provided with a light modulator.
2. Description of the Related Art
As an external modulator for changing the light intensity of a semiconductor laser, it is known which is made by forming an n-InP cladding layer 2, InGaAsP quantum well layer 3, and p-InP cladding layer 4 on an n-InP substrate 1 in order by means of epitaxial growth and forming a p-electrode 5 on the cladding layer 4 and an n-electrode 6 under the substrate 1 as shown in FIG. 1A. The external light modulator changes the absorption wavelength of the light passing through the quantum well layer 3 to perform light modulation by using the characteristic that the light absorption coefficient of the quantum well layer 3 is changed due to a change of the electric field in the quantum well layer 3.
Since this type of light modulator requires a long waveguide, the overall thickness of a device increases by mounting the light modulator on a surface emitting laser.
Therefore the absorption loss increases and the optical output decreases when the electric field of the light modulator is not generated.
A surface emitting laser free from the above problem is described in the official gazette of, for example, Japanese Patent Laid-Open (KOKAI) No. Hei 4-247676. For this apparatus, as shown in FIG. 1B, an n-InP buffer layer 8, saturable absorption layer 9 comprising an n-InGaAs/InP superlattice, n-InP cladding layer 10, active layer 11 comprising an InGaAs/InP superlattice, p-InP cladding layer 12, and p-InGaAs cap layer 13 are formed on an n-InP substrate 7 in order, high-reflectance films 14 and 15 are formed on the cap layer 13 and under the semiconductor substrate 7, a p-electrode 16 is formed around the high-reflectance film 14 on the cap layer 13, an n-electrode 17 is formed on the outer periphery of the saturable absorption layer 9, and moreover an electrode 18 for applying an electric field to the saturable absorption layer 9 is formed on the outer periphery of the buffer layer 8.
This apparatus changes the quantity of light to be absorbed in a resonator by the saturable absorption layer 9 to perform light modulation by using the characteristic that a light absorption wavelength is changed due to the quantum-confinement Stark effect when applying an electric field to the saturable absorption layer 9.
For the structure in which the light modulator is formed in the resonator, however, current also flows to the saturable absorption layer 9 when it flows from the p-electrode 16 to the n-electrode 17 and thus, carrier transition always occurs between a conduction band and a valence band of the saturable absorption layer 9. Therefore, it is difficult to control the absorption wavelength by applying the electric field to the saturable absorption layer 9.